A wound produces a mixture of fluids throughout its healing sequence. This fluid is termed exudate. The exudate's biochemical and physical composition is a function of wound type and its position in the healing sequence. Exudate may range from blood and serous fluids to highly viscous proteinaceous liquids. Exudate is beneficial to the wound repair process and contains the cellular and enzymatic materials beneficial to wound healing. The type of wound dictates such parameters as exudate production and speed of healing, etc.
Wounds can be categorized into two broad types: wounds without tissue loss; and wounds with tissue loss.
Wounds without loss of tissue are typically incision wounds formed either as a result of surgery or intro cut.
Wounds which result in the loss of tissue may be the result of trauma or as a secondary event in chronic ailments, e.g., vascular insufficiency, diabetes, etc.
Iatrogenic wounds may also result in the loss of tissue. This is exemplified in such wounds as skin graft donor sites, dermabrasions, etc.
For the purposes of designing a wound dressing, acute and chronic wounds with significant loss of tissue are a challenging area.
Wounds that have significant tissue loss are: dermal ulcers (venous stasis, diabetic and pressure sores), abrasions (traumatic and iatrogenic), donor sites, and burns.
Dermal ulcers are the result of a patient's underlying physical condition. Venous stasis and diabetic ulcers are a direct result of a degeneration of the cardiovascular system. This leads to reduced blood flow in the extremities and subsequent tissue necrosis resulting in the formation of dermal lesions. Pressure sores, or decubitus ulcers, are formed when skin is subjected to unrelenting pressure and abrasion. These factors induce tissue necrosis and ulceration.
Abrasions arise due to trauma, as in the case of "road rash", or from elective procedures such as dermabrasion. These wounds initially produce copious amounts of exudate composed of blood and serous fluid. Traumatic abrasions are frequently contaminated with physical debris which if unattended to will lead to infection.
Donor sites are created by the removal of a thin layer of skin which is utilized as a "skin graft." As in the case of abrasions, donor sites exude blood and serous fluid. These wounds are painful and often require the patient to undergo painkilling treatment.
Burn wounds cover a range of severity, from superficial to full-thickness. Patients suffer fluid loss from their wounds which must be adequately controlled. Severely burnt patients often become immunosuppressed leaving them vulnerable to infection.
Conventionally, wound dressings have been designed and introduced into the market predominantly to absorb the exudate expected from a particular wound. Therefore, a problem arises if a wound dressing which was designed for use on a wound producing a large amount of exudate is used on a wound which produces little exudate. The latter type wound can become desiccated, resulting in a clinically undesirable situation.
The management of exudate, therefore, is of primary concern in making a wound dressing. It has now become an accepted clinical fact that wounds need to remain moist to optimize healing from the standpoint of rate of healing, quality of healing, with minimal or no scarring, etc. As this practice of "moist wound" healing has grown, the need for wound dressings which provide and promote a controlled, moist wound microenvironment has increased.
It is difficult in the clinical environment to choose a dressing having a uniform level of exudate management capability. This decision is compounded by the fact that wounds, during their healing process, can moderate their exudate level. For example, some wounds can produce copious, high levels of exudate during the first few hours or days after injury but subsequently substantially reduce exudate production.
Ideally, modern synthetic wound dressings should also be provided having a structure which allows the dressing to be left on the wound for prolonged periods of time, e.g., about 3 to about 7 days. Therefore, there is a need and desire for a wound dressing capable of accommodating varying degrees of exudate while maintaining a consistent moist wound healing environment.
In particular, differential control of wound exudate is highly desirable if a moist occlusive wound microenvironment is to be maintained. It can be appreciated that if a dressing removes all the exudate that a wound produces, a "dry" wound results and a condition suboptimal for wound healing arises. Similarly, if the dressing does not control the level of exudate sufficiently, then an excess "pool" of exudate can collect which can subsequently leak thus soiling clothing, bed linen, and also breaching any protective barrier to bacterial infection of the wound.
Ideally, a wound dressing also adhesively attaches itself to the wound site. The adhesive utilized must be biocompatible, non-cytotoxic and free of toxic leachable substances, as well as have the desired balance of physical properties such as moisture vapor transport rate, tack, long term adhesion properties, etc. Inasmuch as in use the adhesive will be in direct contact with the wound site and surrounding intact area, it must be physiologically non-toxic and should elicit no more than a minimal allergenic response.
An ideal wound dressing also provides a barrier preventing bacteria from entering the wound through the dressing from the ambient environment while providing the proper moisture vapor transport rate. Other desirable aspects include a dressing's ability to conform to irregular contours of the body, to be self supporting whether wet or dry, and allow passage of gases from the wound. This may be accomplished in part by utilizing elastomeric, flexible, polymeric materials in the construction of the dressing.
Having outlined the major desirable design characteristics of environmental wound dressings it is beneficial to examine the mode of operation of conventional wound dressings to appreciate their deficiencies.
Conventional wound dressings can be categorized into several broad classes: hydrocolloid dressings; film dressings; foam dressings and gel dressings. These dressings maintain specific microenvironments, e.g., moisture, temperature, gaseous transport, etc., around a wound by utilizing a variety of physical mechanisms.
Traditionally, wound dressings have been categorized by determining their capacity to absorb exudate. This has been routinely accomplished by performing laboratory trials in which dressings are immersed in liquids and the quantity of liquid absorbed quantified.
An important clinical property in all occlusive wound dressings is its moisture vapor transmission rate (MVTR), which is the rate at which moisture permeates through the dressing. MVTR is typically measured and expressed in grams per square meter per 24 hour day (g/m.sup.2 /24 hrs). A conventional dressing has a fixed MVTR regardless of exudate level.
More recently, a new class of environmental wound dressings, "spyrosorbent dressings", have been created. The term "spyrosorbent" is defined as breathable absorbent. Unlike conventional wound dressings, a spyrosorbent dressing has a differential MVTR capability. Spyrosorbent wound dressings not only manage exudate by absorption but have the ability to adjust their moisture vapor transport properties in response to the level of exudate available. That is to say, spyrosorbent dressings possess a level of active intelligence due to their physical and chemical structure.
Conventional dressings also differ markedly from one another in their attributes, such as conformability, adhesiveness, and ease of use. They also differ dramatically in the mechanisms by which they seek to manage exudate.
Film dressings are typically relatively thin films, which utilize exclusively the moisture vapor transport properties of the film materials from which they are constructed. Film dressings are conformable, but on moderately to highly exuding wounds the exudate tends to collect under such film dressings and form "pools". This collection of exudate indicates that the MVTR of conventional polymer film dressings is too low to handle the exudate from many wounds. It has also been suggested that the "pool" of exudate may increase the risk of bacterial proliferation leading to infection. Similarly, if the "pool" reaches excessive proportions, leakage will occur, thus breaking the bacterial barrier. The thin film category of dressings has also shown, however, that by suitable choice of film thickness and molecular structure of the film, MVTR can be substantially increased or reduced dependent upon the requirements of the dressing.
Hydrocolloid and gel dressings all utilize absolute absorption mechanisms by which to manage exudate. As a result of this absorption, they generally tend to be relatively thicker dressings, and less conformable than the film dressings. This can cause a series of problems when utilized in a clinical environment. For example, the ability for moisture to pass through the dressing to the external environment is minimal. On highly exuding wounds, the dressing's absorption capacity can be exceeded leading to leakage and subsequent disruption of the bacterial barrier. Some hydrocolloid compositions can dissolve and enter into the wound bed itself, thus requiring time consuming cleaning, which disrupts the wound site, at subsequent dressing changes.
Alginate dressings, a subset of gel dressings composed of alginate, are examples of dressings which are sometimes supplied as dehydrated or partially hydrated structures. On application to the wound and subsequent absorption of exudate such dressings undergo gelation. However, the swelling of the dressing results in the dressing structure moving away from the wound bed and providing potential air spaces and pockets in which bacteria may proliferate. Moreover, dressings which are supplied in a partially hydrated state are not supplied with a pressure sensitive adhesive coating. Further taping or application of a secondary dressing is therefore required to assure adequate attachment.
Alginate dressings are supplied as a dry, fibrous, mat structure. Alginate dressings are capable of absorbing large quantities of exudate. During absorption they undergo a gelation reaction due to the interchange of sodium and calcium ions between the exudate in the wound bed and the alginate material in the dressing. Alginate dressings, like other gel dressings, require the use of secondary dressings to secure them.
Gel dressings are generally water or saline hydratable or swellable gel (hydrogel) materials supplied on a moisture impermeable polymeric backing sheet. The backing sheet prevents the hydrogel from dehydrating and desiccating the underlying wound. These gel materials have little or no vapor transport capacity. In some instances, it is recommended that the impermeable backing sheet of the gel dressing be removed during the healing sequence, especially on heavily exuding wounds. The removal of the sheet encourages the dehydration of the hydrogel. This, in turn, increases the dressing's ability to handle high levels of exudate. During the dehydration, however, the gel dressing becomes noncompliant, resulting in damage to the underlying wound.
Gel wound dressings, in general, do not dissolve and contaminate the wound and, when hydrated, are more conformable than hydrocolloid dressings but less conformable than thin film dressings.
Foam type dressings utilize both a moisture vapor transport and absorption mechanism by which to manage exudate. These dressings, due to their chemical nature and high degree of hydrophilicity, however, tend to swell and lose mechanical integrity when wet.
Foam dressings manage exudate by evaporation of the aqueous portion of the exudate through the dressing to the surrounding environment. The control of MVTR is a function of the chemical composition of the foam coupled with the pore structure. Due to the gross pore sizes of conventional foams, however, foam dressings tend to desiccate wounds resulting in dressings which become brittle and non-conformable during use. These hardened dressings often traumatize the underlying healing wound bed. In addition, either special processing or a wetting agent, or both, are required to make the foam hydrophilic.
Dependent upon the type of foam structure used, exudate can also be managed by capillary action into the pores of the structure. Most conventional foam materials used as dressings contain interconnecting pores and thus provide limited bacterial barrier properties because the mean pore diameter exceeds the dimensions of many bacteria. Similarly, such dressings contain pore sizes which are sufficiently large as to fall into the range of sizes into which regenerating tissue will grow. As a result, ingrowth of tissue into the dressing's structure occurs thus impeding removal of the dressing and traumatizing the wound site.
Some attempts have been made in the past to overcome the foregoing deficiencies of film dressings and, in particular, the "pooling" of wound exudate.
Polymer film dressings as described in U.S. Pat. No. 3,645,835 to Hodgson and. U.S. Pat. No. 4,513,739 to Johns are thin and possess high conformability. The wound contacting surfaces are coated with pressure sensitive adhesives carried on the film. The films that are used are liquid impermeable poly urethane elastomers. Thus wound exudate cannot enter into the film. The sole mode of exudate control is by allowing the vapor of the aqueous portion of the exudate to permeate into the polymer film from where it diffuses into the external environment. As the moisture vapor permeability is low, however, the polymer film's absolute absorption capacity is also low, especially when compared to hydrocolloid dressings.
In U.S. Pat. No. 4,747,401 and U.S. Pat. No. 4,595,001, both to Potter et al., a surgical wound dressing is described composed of a continuous hydrophilic film laminated to a discontinuous adhesive layer. The film is selected to have a higher moisture vapor permeability when it is in contact with water than with moisture vapor. The moisture vapor permeability of the laminate dressing is stated to be not more than 2000 g/m.sup.2 /24 hrs when the adhesive layer is in contact with water vapor but not liquid water, and not less than 2,500 g/m.sup.2 /24 hrs when the adhesive layer is in contact with liquid water. However, a MVTR of not more than 2,000 g/m.sup.2 /24 hrs is undesirably low for the management of moderate to heavy exudate. Moreover, the chemical and laminate composition of the polymer films described structurally restricts the degree of differential MVTR which might be attainable with such a dressing structure. Thus, spyrosorbent wound dressings which simultaneously balance moisture vapor transport and absorption mechanisms within one dressing are desirable and needed. In particular, an ideal spyrosorbent dressing would have a low profile, be highly conformable, breathable and absorptive and not become exhausted or have a finite exudate management capacity.
This need has been satisfied in part by the wound dressings described in U.S. Pat. No. 4,906,240 and U.S. Pat. No. 5,098,500, both to Reed, et al., owned by the assignee of the present invention. The disclosures of the foregoing patents are incorporated herein by reference. Sheet-form wound dressings are described which comprise a porous sheet of absorbent elastomeric segmented polyurethane having an open pore size gradient, such that larger pores are away from the wound side, and an apertured adhesive facing contiguous with the large pore surface. This apertured adhesive structure enhances the management of exudate by providing fluid channels for capillary transport of proteinaceous exudate to the interior of the porous sheets. By varying the chemical composition of the film layer, the MVTR of the film and, thus of the wound dressing, can be adjusted as desired.
One spyrosorbent environmental membrane laminate dressing has been developed and sold under the trademark, MITRAFLEX.RTM. by PolyMedica industries, Inc. (Golden, Colo.), the assignee of this invention. Briefly described, this dressing is a trilaminate structure of porous, pressure sensitive adhesive attached to an absorptive, polyurethane, microporous membrane which is laminated to a thin, transparent, hydrophilic, polyurethane film. This spyrosorbent wound dressing is used to manage exudate from dermal ulcers, skin donor sites, superficial burns, abrasions and lacerations.
A description of the development and properties of the MITRAFLEX.RTM. dressing can be found in the article by Reed, Andrew M., "Mitraflex: Development of an intelligent Spyrosorbent Wound Dressing," Journal of Biomaterials Applications 6: 3-41, Technomic Publishing Co., Inc. (1991).
The spyrosorbent wound dressings of the present invention further satisfy the ongoing need for exudate management by providing dressing structures having differential MVTR properties and improved exudate management capability. Such structures desirably adjust a dressing's exudate transport away from the wound site in response to the quantity of exudate produced by a wound and modulate the rate of exudate transport.